Metallic copper microparticles are inexpensive materials having good electric conductivity and are widely used as materials for ensuring electric conduction, such as circuit forming materials for printed-wiring boards, various electrical contact materials, outer electrode materials of capacitors, etc., and, recently, they are used also for inner electrodes of laminated ceramics capacitors. The laminated ceramics capacitors can give a larger capacity as compared with capacitors of other types, such as electrolytic capacitors and film capacitors, and are superior in mountability and high in safety and stability, and thus they have spread rapidly. With recent miniaturization of electronic equipments, the laminated ceramics capacitors are also being miniaturized. However, for maintaining large capacity, it is necessary to miniaturize them without reducing the number of lamination of ceramics sheet, and there is a limit in thinning of the sheet from the point of strength. Therefore, the miniaturization of laminated ceramics capacitors is realized by thinning the inner electrodes using fine metallic copper particles.
In order to use metallic copper microparticles as materials for ensuring electric conduction, usually copper microparticles are dispersed in a solvent or mixed with a binder such as epoxy resin thereby to prepare a paste, paint or ink, namely, a fluid composition such as a copper paste, paint or ink. For example, in forming a circuit of a printed-wiring board, the fluid composition is coated on a substrate in the pattern of circuit or electrode by means of screen printing, ink jet printing, or the like, and then heated to fusion bond the metallic copper microparticles, thereby forming fine electrodes. Furthermore, for formation of an inner electrode of laminated ceramics capacitors, the fluid composition is coated on thin ceramics sheets, and these sheets are laminated and then fired by heating to form inner electrodes.
As a process for producing metallic copper microparticles, there is known a process of reducing copper oxide with a hydrazine reducing agent in an aqueous liquid medium containing a protective colloid such as gum arabic (Patent Document 1), and in an example of this process, there are obtained copper microparticles having an average particle diameter in the range of 0.4-32 μm as measured by an electron microscope. There is further known a process which comprises precipitating copper hydroxide from an aqueous solution of a copper salt, adding a reducing sugar to the resulting copper hydroxide to reduce it to copper suboxide (cuprous oxide), and then reducing the cuprous oxide to metallic copper with a hydrazine reducing agent (Patent Document. 2). This document discloses that when a complexing agent such as Rochelle salt, amino acid, ammonia or an ammonium compound is added to the aqueous solution of copper salt, copper ion can be stably dissolved, and hence the addition of complexing agent is effective. In an example of this process, copper microparticles having a particle size distribution of 0.5±0.1 μm to 3.1±0.3 μm are obtained. Another process is known according to which in the process of Patent Document 2, gelatin as a protective colloid is added to the aqueous solution of copper salt and is further added during subsequent growth of nucleus particles of metallic copper (Patent Document 3). In an example of this process, there are obtained copper microparticles having a particle size distribution of 0.90±0.05 μm to 5.14±0.25 μm as measured by microtrack (dynamic light scattering method).
Patent Document 1:JP 61-55562 B2Patent Document 2:Japanese Patent 2638271Patent Document 3:JP 4-235205 A